The invention relates to a process for producing crack-free shaped ceramic bodies based on Si/C/N by hot pressing of crosslinked polysilazane powder and subsequent pyrolysis of the hot-pressed shaped body, and also to the use of a shaped body produced in this way for tribological purposes.
In the case of materials for tribological applications, for example brake linings, it is not only the friction properties but also, in particular, the wear behavior of the material which is of interest. Here, both the wear of the part itself and also the wear caused on the other part against which rubbing occurs have to be taken into account. Both should be as small as possible. However, in the case of wear parts such as brake linings, minimum wear on the part against which rubbing takes place, for example a brake disk, is of even greater importance.
Ceramics produced by the polymer pyrolysis process have great advantages in this respect. Such ceramic materials based on Si/C/N can be produced by pyrolysis of organic polymers without addition of additives (R. Rice, Ceram. Bull. 62 (1983), 889; D. Seyferth et al., J. Am. Ceram. Soc. 67 (1984), 132). In this production method, the polymer firstly has to be optimally crosslinked by thermal means in order to be able to be converted into dense shaped parts by subsequent plastic deformation. The pressing temperature must not exceed the temperature maximum in the TMA curve (TMA=thermomechanical analysis), since otherwise cracks can arise in the pyrolyzed ceramic body formed (J. Seitz et al., J. Mater. Sci. Letters 15 (1996), 391-393; conference proceedings xe2x80x9cReibung und Verschleixcex2xe2x80x9d, DGM, Bad Nauheim (1996), 335-340). In order to achieve optimum crosslinking, the crosslinking procedure has to be matched individually to each polymer batch, which is laborious.
It is therefore an object of the invention to provide a process for producing crack-free shaped ceramic bodies based on Si/C/N which does not have the disadvantages of the prior art.
This object is achieved according to the invention by a process for producing crack-free shaped ceramic bodies based on Si/C/N by hot pressing of crosslinked polysilazane powder and subsequent pyrolysis of the hot-pressed shaped body, wherein the pressing temperature employed is higher than the temperature maximum of the TMA curve of the optimally crosslinked polymer.
It has surprisingly been found that the process of the invention makes it possible to obtain ceramic materials which have excellent properties, for example a low coefficient of friction, homogeneous frictional behavior and, in particular, low wear values, specifically in respect of the wear of the part against which rubbing takes place. Furthermore, the polymer pyrolysis ceramic materials produced using the process of the invention display excellent thermal stability and oxidation resistance.
According to the invention, crosslinked polysilazane powders are used as starting materials for producing crack-free shaped ceramic bodies based on Si/C/N. The polysilazane powder is selected according to the respective requirements of the intended application. Suitable polysilazanes include, for example, polyhydridomethylsilazanes, polyvinylsilazanes and others. However, it is also possible to use modified polysilazanes which contain further elements, in particular titanium and/or boron, in addition to Si/C/N. Such modified polysilazanes can be prepared, for example, by reaction of polysilazanes with element alkylamides (thesis, M. Friexcex2, Stuttgart, 1994) or by hydroboration of vinylsilazanes. The polysilazane powder used is particularly preferably a polyhydridomethylsilazane marketed under the name NCP 200 by Nichlmen Corp. or a polyvinylsilazane marketed under the name VT 50 by Hoechst AG.
According to the invention, the polysilazane powder is firstly crosslinked. Here, it is not necessary to optimize the crosslinking individually for each polymer batch. For the purposes of the process of the invention, it is sufficient to carry out the crosslinking using a standard crosslinking program regardless of the polymer powder used. For the purposes of the present invention, an optimally crosslinked polymer is a material whose degree of crosslinking makes it possible to produce crack-free, abrasion-resistant shaped parts by cold isostatic pressing and subsequent pyrolysis. Thus, an xe2x80x9coptimally crosslinked polymerxe2x80x9d is suitable for cold isostatic pressing.
The crosslinked polysilazane powder is pulverized if necessary and hot pressed. Subsequent pyrolysis gives dense shaped parts. These shaped parts typically have a density of greater than 90% of the theoretical density, preferably greater than 93% of the theoretical density and particularly preferably greater than 95% of the theoretical density. It has surprisingly been found that hot pressing at elevated temperature makes it possible to obtain crack-free shaped ceramic bodies without optimized thermal crosslinking having to be carried out beforehand. According to the invention, the pressing temperature during shaping is selected so that it is higher than the temperature maximum of the TMA curve of the optimally crosslinked polymer.
In TMA (thermomechanical analysis), a specimen is heated in an inert atmosphere at a predetermined heating rate while the linear shrinkage or expansion is measured. The heating rate is typically 5 K/min. For evaluation of the test, the relative shrinkage in % is plotted against the temperature. The temperature value at which the recorded curve has its maximum is referred to as the temperature maximum. The TMA curve usually rises initially with steadily increasing temperature to a temperature maximum and then falls as the temperature is increased further.
According to the invention, hot pressing is carried out at a temperature which is higher than the temperature maximum of the TMA curve of the optimally crosslinked polymer. The temperature employed is preferably from 50 to 1000xc2x0 C. higher than the temperature maximum of the TMA curve. Hot pressing is preferably carried out at a pressure of from 10 MPa to 70 MPa, particularly preferably from 20 MPa to 60 MPa and most preferably from 30 MPa to 50 MPa. Hot pressing can be carried out at a pressing temperature up to the pyrolysis temperature of the respective materials (e.g. 1050xc2x0 C. for NCP 200). The pressing temperature employed for hot pressing is preferably from 275 to 600xc2x0 C., particularly preferably from 300 to 500xc2x0 C. and most preferably from 330 to 450xc2x0 C. Polyhydridomethylsilazane is, after thermal crosslinking and pulverization, preferably pressed to form shaped bodies at a temperature of from 350 to 420xc2x0 C.
Pyrolysis of the hot-pressed shaped bodies, which are also referred to as green bodies, gives ceramics having a high density. The density of the ceramics is generally  greater than 92% of the theoretical density, preferably  greater than 95% of the theoretical density and particularly preferably  greater than 97% of the theoretical density. Depending on the conditions employed for the thermal treatment, amorphous or crystalline structures are obtained. Amorphous structures are obtained particularly when the pyrolysis is carried out in a temperature range from about 800 to 1200xc2x0 C., preferably from 1000 to 1200C. When the thermal treatment is carried out at higher temperatures, for instance from 1200 to 2000xc2x0 C., preferably from 1500xc2x0 C. to 2000xc2x0 C., at least partially crystalline structures are obtained. The hot pressing and/or the pyrolysis are preferably carried out in an inert atmosphere, for example under argon or nitrogen.
Studies on crack-free shaped ceramic bodies produced according to the invention showed that the materials tested, in particular amorphous ceramics produced from NCP 200 or VT 50, are superior to the previously known tribological materials and result in minimal wear on the part against which rubbing takes place.
The pyrolysis of the pressed body is preferably carried out at a temperature of from 1000 to 1200xc2x0 C. under protective gas, for example under argon or nitrogen. In the case of phase separation in the temperature range from 1000 to 2000xc2x0 C., precipitation of lubricating substances such as graphite or, if the shaped body contains a further element, e.g. titanium or boron, in addition to the elements Si/C/N, hexagonal boron nitride or titanium carbonitride can occur, depending on the composition of the respective material. These precipitates serve to distribute these materials, which are of interest primarily in respect of solid-state lubrication of the ceramic materials, uniformly in the ceramic material. In the case of titanium-containing materials, the formation of nonstoichiometric titanium oxides, so-called xe2x80x9clubricious oxidesxe2x80x9d, which have lubricant properties from titanium carbonitride is possible when the part is used in a wear situation. Titanium-containing polysilazanes can be prepared, for example, by reacting polysiloxanes with titanium alkylamides, for example the tetrakis(dialkylamino)titanium compounds Ti[N(CH3)2]4 or Ti[N(CH2CH3)2]4, with the crosslinking of the polysilazane being carried out, in particular, by reaction with such a tetrakis(dialkylamino)titanium compound. It is also possible to use a crosslinked polysilazane powder which is obtained by hydroboration of vinylsilazane to form a boron-containing polysilazane.
The invention further provides for the use of a shaped body produced by the process of the invention for tribological purposes. The shaped bodies produced according to the invention display excellent frictional properties such as low coefficients of friction ( less than 0.2, preferably  less than 0.1), homogeneous frictional behavior and particularly low wear values and also a high thermal stability and oxidation resistance. The shaped bodies produced according to the invention have, in particular, a coefficient of friction which lies within a predetermined range (preferably xc2x120%, particularly preferably xc2x110% and most preferably xc2x15%) and displays only slight fluctuations, so that uniform braking action is ensured. The material properties can be matched very well to the respective requirements of the tribological application by variation of the crosslinked polymer material used. In particular, the shaped bodies produced according to the invention are suitable for producing brake materials, e.g. brake linings, which cause extremely low wear on the brake disk.